I. Probing Ion Channel Functional Structure with Water-Soluble Soft Polymers. To understand the physics of polymer equilibrium and dynamics in the confines of ion channel pores, we study partitioning of polyethyleneglycols (PEGs) of different molecular weights into the bacterial porin, OmpF. Thermodynamic and kinetic parameters of partitioning are deduced from the effects of polymer addition on ion currents through single OmpF channels reconstituted into planar lipid bilayer membranes. The equilibrium partition coefficient is inferred from the average reduction of channel conductance in the presence of PEG; rates of polymer exchange between the pore and the bulk are estimated from PEG-induced conductance noise. Partition coefficient as a function of polymer weight is best fitted by a 'compressed exponential' with the compression factor of 1.65. This finding demonstrates that PEG partitioning into OmpF channel pore has sharper dependence on polymer molecular weight than predictions of either hard-spheres, random-flight, or scaling models. A 1360 Da polymer separates regimes of partitioning and exclusion. Comparison of its characteristic size with the size of a 2200 Da polymer previously found to separate these regimes for the a-toxin shows good agreement with the X-ray structural data for these channels. The PEG-induced conductance noise is compatible with the polymer mobility reduced inside the OmpF pore by an order of magnitude relatively to its value in bulk solution. II. Channel-Facilitated Metabolite Transport. Nucleotide penetration into the voltage-dependent mitochondrial ion channel, VDAC, reduces single-channel conductance and generates excess noise in the ionic current through a fully open channel. At a given nucleotide concentration the average decrease in small-ion channel conductance induced by mononucleotides ATP, ADP, AMP, and UTP and dinucleotides beta-and alpha-NADH, NAD, and NADPH are very close. However, the excess current noise is about 7 times higher in the presence of NADPH than in the presence of ATP and is about 40 times higher than in the presence of UTP. The nucleotide-generated low-frequency noise obeys the following sequence: NADPH, NADH, ATP, ADP, NAD = AMP, UTP. Measurements of bulk-phase diffusion coefficients and of the effective charge of the nucleotides show that differences in size and charge cannot be the major factors responsible for the ability to generate current noise. Thus, while the ability of nucleotides to partition into the channel's pore is very similar, the ability to generate current noise reflecting their mobility and interaction with the channel pore involves a detailed recognition of the three-dimensional structure of the nucleotide by the VDAC channel.